Technical Field
This document discloses an internal gear pump for high speed pumping of liquids. The pump features a head that features rotor feed slot that directs flow from the inlet to the rotor teeth, an idler feed slot that directs flow from the inlet to the idler teeth and a crescent configuration that includes a liquid directing step that separates the idler and rotor feed slots. The crescent further includes a tapered leading edge. The pump also features a casing with symmetrical porting that enables the flow direction to be reversed by replacing only the head.
Description of the Related Art
Internal gear pumps are capable of efficiently pumping low to moderate viscosity liquids at relatively high speeds. A typical internal gear pump includes a rotor mounted to a shaft. The rotor includes a plurality of circumferentially disposed, spaced-apart and inwardly directed rotor teeth that also extend axially toward an open end of the pump casing. A head covers the open end of the pump casing and the head connects to an idler by an idler pin mounted to the head eccentrically with respect to the shaft and rotor teeth. The idler includes a plurality of idler teeth disposed between alternating idler roots. In contrast to the rotor teeth, which taper as they extend radially inward, the idler teeth taper as they extend radially outward.
A crescent or sealing wall is disposed below the idler and radially within the rotor teeth. The crescent directs some of the incoming liquid to the idler teeth and some of the incoming liquid to the rotor teeth. The rotor teeth rotate below or along a lower side of the crescent while the idler teeth rotate above or along an upper side of the crescent before the idler and rotor teeth rotate past the outlet and intermesh with each other at the top of the pump. The crescent provides as seal between the outlet and inlet as an idler tooth engages the upper side of the crescent and as a rotor tooth engages the lower side of the crescent. Further, as the idler and rotor teeth intermesh at a position opposite the idler pin from the crescent (and generally equidistant from the inlet and outlet), the intermeshed idler and rotor teeth also act as a seal between the inlet and outlet. These seals help to force the liquid out of the pump chamber through the outlet and help to reduce slip, or the migration of liquid from the outlet back to the inlet. Because slip results in liquid being recycled within the pump, it reduces the pumps total flow rate and therefore the efficiency of the pump.
Incoming liquid from the inlet flows either to spaces between the rotor teeth prior to the rotor teeth rotating along the lower side the crescent or to roots disposed between adjacent idler teeth prior to the idler teeth rotating along the upper side of the crescent. The roots between the idler teeth may be loaded in two ways: radially and axially. Radial loading of the idler teeth occurs when fluid passes between adjacent rotor teeth before flowing into a root disposed between adjacent idler teeth. Axial loading of the idler teeth occurs when liquid, disposed in an area between the head and the idler, flows axially into a root as the idler and rotor teeth rotate from an intermeshed position and towards the inlet.
It is difficult to ensure a complete loading of the idler roots. The failure to provide a complete loading of the idler roots reduces pump efficiency. Similarly, it is difficult to ensure a complete loading of the spaces between the rotor teeth. The failure to provide a complete loading of rotor teeth also results in reduced pump efficiency. Therefore, there is a need for a way to improve the loading of the idler roots and/or the spaces between rotor teeth of internal gear pumps as a means for increasing pump efficiency.
Cavitation describes the phase change from liquid to gas (boiling) that occurs in a pump when the inlet pressure falls below the vapor pressure of the liquid being pumped, thereby causing vapor bubbles. Because vapor bubbles take up more volume than the liquid, a reduction in liquid flow occurs. As the vapor bubbles move from the inlet of the pump towards the roots of the idler teeth, the bubbles collapse back into the liquid phase and, at the moment of collapse or implosion, a powerful shockwave develops within the liquid. This shockwave can damage the idler, creating pits. In an internal gear pump, cavitation can be caused by operating the pump at high speeds. Specifically, as the idler and rotor teeth move from an intermeshed relationship at a position opposite the idler pin from the crescent to a separated relationship at the inlet, a low-pressure condition can develop which can lead to cavitation. While reducing the pump speed can alleviate this problem, reducing the pump speed also reduces the pump output, which can be disadvantageous. Because increasing the pressure of the liquid delivered to the inlet may not be an option, there is a need for an improved internal gear pump designs, which permit high-speed operation of the pump while limiting the effects of cavitation. Further, there is a need for improved internal gear pumps wherein the speed at which cavitation begins to occur is higher than in currently available designs.